glsl: Fix 'control reaches end of non-void function' warning.

Fixes this GCC warning.

lower_variable_index_to_cond_assign.cpp:
In member function
'bool variable_index_to_cond_assign_visitor::needs_lowering(ir_dereference_array*) const':

lower_variable_index_to_cond_assign.cpp:261:
warning: control reaches end of non-void function

src/glsl/lower_variable_index_to_cond_assign.cpp

@@ -258,6 +258,7 @@ public:
       }
 
       assert(!"Should not get here.");
+      return false;
    }
 
    ir_variable *convert_dereference_array(ir_dereference_array *orig_deref,